A relay laser communication coding method for FSO systems

Abstract

The present invention belongs to the field of free-space laser communication, and in particular, is a relay laser communication coding method for FSO systems, comprising the following steps: Step 1, obtaining a sequence to be coded at a source node, and polarization coding of K=k+m codewords as information bits; Step 2, using an interleaver to change the information position to the maximum extent and resist long-segment errors; Step 3, sending the codeword sequence at the source node to the destination node and the relay node via BPSK modulation. The present invention integrates cooperative communication with channel coding technology to achieve efficient cooperative transmission of Polar codes in a cooperative relay system to achieve the capacity of the relay system. A single link cannot resist channel interference well, and cooperative technology can effectively combat fading, thereby improving the ability of the relay system to resist continuous burst errors, and combining channel coding and cooperative diversity technology to achieve the capacity of the relay system.

Description

Technical Field

[0001] The present invention relates to the technical field of free space laser communication, and in particular to a relay laser communication coding method for an FSO system. Background Art

[0002] As space exploration continues to develop, high-speed communication systems are increasingly needed. The channel state of free-space optical communication (FSO) changes over time, and communication is affected by atmospheric attenuation and turbulence, which degrades system performance and limits transmission distance. Severe attenuation and light intensity fluctuations cause light intensity flicker, inter-symbol crosstalk, and beam expansion, resulting in signal energy loss, a reduction in the received signal-to-noise ratio, and significantly reduced communication link reliability.

[0003] Polar codes, due to their numerous advantages, have become a research hotspot in the field of channel interference-resistant coding. The outstanding performance of polar codes is primarily reflected in long codes; the polarization effect increases with increasing code length. However, in practical applications, code length can limit decoding performance.

[0004] Chinese patent application publication number CN114448567A discloses a polar coding cooperative communication method for FSO systems. This method uses polar coding-based cooperative communication and sets a forwarding threshold, improving the system outage probability and bit error rate. However, it does not employ interleaving, and thus fails to address the long-slice bit error problem encountered in fading channels.

[0005] Chinese patent application publication number CN110213016B discloses a multi-relay selective polarization decoding and forwarding method. This method constructs a polarization code with a flexible and variable code rate based on polarization channel state information, uses the optimal relay to forward information, and combines channel coding with multi-relay collaborative decoding to achieve efficient transmission. However, it still does not address the long-segment bit error problem caused by transmission in FSO channels.

[0006] In the free-space relay laser communication scenario, there are currently very few coding cooperation schemes that can effectively improve the polarization code encoding and decoding performance under fading channels. In order to meet the high-speed data transmission requirements of free-space relay communication, coding cooperation communication has become a key technology.

[0007] A free-space laser relay communication system consists of a source node (SN), a destination node (DN), and a relay node (RN). This paper focuses on improving decoding performance for limited code lengths by constructing an interleaved relay coding cooperative system based on the Plotkin architecture. This cooperative communication approach can suppress the interference of atmospheric turbulence on the communication system's bit error rate performance, further improving the system's transmission reliability within the channel. Designing an effective and high-performance coding cooperative communication scheme is of great significance in the field of wireless laser communications. Summary of the Invention

[0008] (1) Technical problems solved

[0009] In view of the shortcomings of the existing technology, the present invention provides a relay laser communication coding method for FSO systems, which solves the problem of optical signal attenuation caused by atmospheric turbulence in relay laser communication.

[0010] (2) Technical solution

[0011] In order to achieve the above-mentioned purpose, the present invention specifically adopts the following technical solutions:

[0012] A relay laser communication coding method for an FSO system includes the following steps:

[0013] Step 1: Obtain the sequence to be encoded at the source node, and use K = k + m codewords as information bits for polar coding;

[0014] Step 2: Use an interleaver to change the information position as much as possible to resist long-segment errors;

[0015] Step 3: The codeword sequence at the source node is BPSK modulated and sent to the destination node and relay node;

[0016] Step 4: The relay node performs SCL decoding on the received optical signal and extracts the information estimation sequence;

[0017] Step 5: Use the Plotkin structure to construct the generator matrix of two codewords for polar coding;

[0018] Step 6: The destination node combines the two signals with equal gain, deinterleaves them, and sends them to the decoder for error correction.

[0019] Furthermore, the method for obtaining the sequence to be encoded and the information bits at the source node includes:

[0020] Step 1: Obtain the sequence to be encoded at the source node, determine the subchannel for transmitting information bits based on the Bhattacharyya parameter size and code rate, perform reliability sorting, and obtain an information sequence of length k;

[0021] Step 2: Add a CRC check code to the information bit, and the resulting K = k + m-bit codeword is used as the information bit of the polar code for polar coding and sent to the encoder.

[0022] Furthermore, the method of using an interleaver at the source node includes:

[0023] Step 1: After polarization encoding, the codeword sequence passes through a block interleaver to maximize the change of information position and resist long-segment errors. After BPSK modulation, the codeword sequence is sent to the channel.

[0024] Step 2: Under the assumption of a turbulent channel, the error probability upper limit of the coding scheme is calculated, and the interleaving depth is adjusted based on the approximate error upper limit of the paired codewords.

[0025] Furthermore, the method of modulating the sequence at the source node and sending it to the destination node and the relay node includes:

[0026] Step 1: Two different channels send different parts of the same information. The information selector extracts the codeword u, which has a length of N / 2 and is sent to the destination node after BPSK modulation.

[0027] Step 2: The other side also modulates the codeword through BPSK and sends it to the relay node.

[0028] Furthermore, the polar coding method using the Plotkin structure to construct a generator matrix of two codewords includes:

[0029] Step 1: Construct two codeword generator matrices for encoding. The first path is to multiply the codeword with the generator matrix G1 to obtain the sequence u1. The other path is to send m1 to the information selector and select the sequence m2 with a code length of k2.

[0030] Step 2: Multiply the m2 sequence by the generator matrix G2 to obtain the sequence v1, perform modulo-2 addition on the information sequence u1 obtained by the first path and v1 obtained by the second path, and finally perform BPSK modulation before sending it to the destination node.

[0031] Furthermore, the destination node combines the two signals with equal gain, deinterleaves the signals, and sends the deinterleaved signals to a decoder for error correction. The method includes:

[0032] Step 1: Receive the signal y1 from the source node and the signal y2 transmitted by the relay node, and combine them with equal gain;

[0033] Step 2: Then send it to the deinterleaver to deinterleave the long continuous error bits, restore the disrupted interleaved sequence to the original sequence, and send the correct sequence after deinterleaving to the error correction code decoding module.

[0034] Furthermore, the method for completing error correction includes:

[0035] Step 1: Convert the long-slice errors caused by atmospheric turbulence into random errors. Calculate the path metric value of each branch, select the largest path as the decoding result, and generate an estimated sequence of the information sequence.

[0036] Step 2: Perform CRC check on the redundant information bits of the transmitted codeword at the same time, delete the check bits of the candidate sequence, and obtain the final decoding result.

[0037] (3) Beneficial effects

[0038] Compared with the existing technology, the present invention provides a relay laser communication coding method for FSO system, which has the following beneficial effects:

[0039] This invention integrates cooperative communication with channel coding technology to achieve efficient cooperative transmission of Polar codes in a cooperative relay system, thereby achieving maximum relay system capacity. While a single link cannot effectively resist channel interference, cooperative technology can effectively combat fading, improving the relay system's ability to withstand continuous burst errors. This combination of channel coding and cooperative diversity technology achieves maximum relay system capacity.

[0040] By implementing interleaving at the source node, long-segment errors are interleaved, improving the relay system's ability to withstand continuous burst errors. The parity bits of some codewords are transmitted using a Plotkin structure. At the destination node, block codes compensate for the reliability of the polar code subchannels, addressing the issue of uneven subchannel reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is the overall flow chart of the relay laser communication encoding method of the present invention;

[0042] Figure 2 This is a schematic diagram of the relay laser communication coding principle of the FSO system of the present invention;

[0043] Figure 3 Schematic diagram of relay laser communication coding of the FSO system of the present invention;

[0044] Figure 4 It is a schematic diagram of the interweaving of isosceles right triangles;

[0045] Figure 5 Schematic diagram of the interleaving module of the present invention. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0047] Example

[0048] like Figure 1 As shown in FIG, a general flow chart of the relay laser communication encoding method according to an embodiment of the present application is shown, and the steps include:

[0049] Step 1: A sequence to be encoded is obtained at a source node, and K=k+m codewords are used as information bits for polarization encoding.

[0050] As in 002, the sequence to be encoded is obtained at the source node The code length is N=2 n , n = [log2M], select the sub-channel for transmitting information bits according to the Bhattacharyya parameter size and code rate, perform reliability sorting, obtain the information sequence of length k, and calculate the Bhattacharyya parameter of each sub-channel using the following formula:

[0051]

[0052] Among them, σ 2 is the variance, y is the received signal, the sub-channel reliabilities are sorted, and the k sub-channels with the smallest Bhattacharyya parameter values ​​are used to transmit information bits, and an information sequence with an input length of k can be obtained.

[0053] Add a CRC check code to the information bit, and the resulting K = k + m codeword is used as the information bit of the polar code and sent to the encoder. Add an m-bit CRC to the k information bits, and set the same polynomial g(x) = x at the sending and receiving ends. n +p1x n-1 +…+p n-1 x+1, in the encoding stage, the information polynomial m(x) is divided by g(x), and the remainder r(x) is used as the m-bit CRC check code.

[0054] The information bit u′ of the polar code is (u1, u2,…, u i ,…u k ,v1,v2,…v i ,…,v m ) is composed of a check code and the original information bits, and the resulting K = k + m codewords are used as the information bits of the polar code, with a code rate of N = K / R.

[0055] The channels are marked as W1, W2, ..., W i ,…,WN , the process of merging and splitting the polar code encoded physical channel W is performed using the linear transformation matrix To express,

[0056] By the formula Calculate the output sequence Among them G N (A) is the sub-matrix of information bits, u A represents the information sequence, u Ac is a fixed bit sequence, and Taken from the reliable polarized channel set A, A={a1,a2,…,a r}.

[0057] Step 2: Use an interleaver to change the information position as much as possible to resist long-segment errors.

[0058] For example, in order to further optimize the coding performance in the fading channel, an interleaver is used to change the information position to the maximum extent and resist long-slice errors. The interleaving matrix is ​​π=[π1,π2,…,π L ] T , yes Random permutation of .

[0059] The total number of interleaved rows is Where E is the target sequence length, and the remaining number of interleaved rows and the number of bits per column are calculated using the following formula:

[0060]

[0061]

[0062] The error probability upper bound of the coding scheme is calculated. Based on the approximate upper bound of the paired codeword error, the interleaving depth is adjusted. Using perfect interleaving can ensure the independence of fading coefficients. The error performance is improved when T / τ0 is large, where τ0 is the coherence time and T is the codeword spacing.

[0063] Under the assumption of turbulent channel, the error probability upper limit of the coding scheme is calculated. Based on the approximate error upper limit of the paired codewords, the interleaving depth is adjusted. There are two codewords. and The symmetric difference of the subset of their state symbol indices is defined, and under the assumption of a turbulent channel, the upper bound is approximated as:

[0064]

[0065] in Signal-to-noise ratio SNRγ=(ηI0) 2 / N0, is the covariance matrix of the state bit sequence, is the logarithmic amplitude sequence, the joint probability density function Use the following formula to calculate:

[0066]

[0067] Step 3: The codeword sequence at the source node is modulated by BPSK and sent to the destination node and relay node.

[0068] For example, in 004, different parts of the same information are sent by two different channel links. The codeword u is extracted by the information selector. The codeword has a length of N / 2 and is sent to the destination node after BPSK modulation. The other side also sends the codeword to the relay node after BPSK modulation.

[0069] For any There is a bit error rate O(·) represents the asymptotic property of the function. The system polar codes of the direct link SD and the SR link are constructed in the same way, and the system frozen bit set is:

[0070]

[0071] Step 4: The relay node performs SCL decoding on the received optical signal and extracts the information estimation sequence.

[0072] As shown in 006, the relay node receives the optical signal sent by the source node, performs bit flipping operation on the sequence and performs SCL decoding, extracts the information estimation sequence, and extracts the index information bit set. After decoding is completed, the estimated sequence of the information is converted into Extract it.

[0073] The information bit u i The corresponding N decision elements are decoded one by one from 1 to N, and the polarization channel The output is and The transition probability function, likelihood ratio formula and estimated values ​​of each bit are as follows:

[0074]

[0075]

[0076] Among them, the information bit decision function is

[0077] By Shiki Calculate the maximum value path in the candidate list, where The corresponding sequence is used as the decoding output, where argmax(·) is the function that maximizes the objective function.

[0078] Step 5: Use the Plotkin structure to construct the generator matrix of two codewords for polar coding;

[0079] For the system polar code encoding that can reach the RD link channel capacity, assume two linear block codes Code1 and Code2, whose parameters are u1∈C1(n1,k1) and v1∈C2(n2,k2), respectively. By performing Plotkin construction on them, we can obtain Code3, which has a longer code length and is also a linear block code:

[0080]

[0081] Where "|" indicates that the information sequence u is concatenated after the information sequence ν+u. The code length of the codeword C3 constructed by Plotkin is N=n1+n2, where n1 is the coding length of u, n2 is the coding length of ν, k1>k2, the number of information bits is K=k1+k2, and the code rate R=K / N=(k1+k2) / (n1+n2).

[0082] Assume that the generator matrices of Code1 and Code2 are G1 and G2, and let Code1 = u A1 G1, Code2 = u A2 G2 will get a longer code length and is also a linear block code x = (Code1 + Code2 | Code1)B N , the generator matrix of G3 codeword can be expressed as follows:

[0083]

[0084] The first path is to multiply the sequence u1 by the generator matrix G1. The codeword information obtained after encoding is the same as the codeword information obtained by encoding the original generator matrix. The other path is to send m1 to the information selector.

[0085] Select the sequence m2 with a code length of k2, multiply the m2 sequence by the generator matrix G2 to obtain the sequence v1, perform modulo-2 addition on the information sequence u1 obtained by the first path and the v1 obtained by the second path, and finally perform BPSK modulation on them before sending them to the destination node.

[0086] Step 6: The destination node combines the two signals with equal gain, deinterleaves them, and sends them to the decoder for error correction.

[0087] For example, at 0010, at the destination node, the signal y1 from the source node and the signal y2 transmitted by the relay node are first received. The signal-to-noise ratio at the receiving end is γ RN =η 2 I2 / N0.

[0088] The information bits of the SR link and the SD link are combined Equal Gain Combining (EGC) is used to send the data to the deinterleaver, which deinterleaves the long continuous bit errors and restores the disrupted interleaved sequence to the original sequence, thus dispersing the sudden errors.

[0089] For example, 0011 is input into the decoder again for decoding to obtain the estimated sequence, and the judgment is m=(m1,m2,…,m N ), the likelihood ratio of the destination node is calculated according to the following formula:

[0090]

[0091]

[0092] The decoder can avoid estimating frozen bits, set the path metric initial value to 1, and calculate the path metric value of each branch. The formula is as follows:

[0093]

[0094] Before calculating the next layer, if the number of paths is greater than L, the paths with smaller path metrics need to be deleted to keep the number of paths at L. Then, the calculation continues to the next layer, and the path with the largest path metric is selected as the decoding result to obtain the final decision sequence d. The actual bit rate is:

[0095]

[0096] After decoding, CRC check is performed to generate information sequence The estimated sequence of Perform CRC check in sequence, and use the candidate codeword r(x) to take the modulus of the generator polynomial g(x). If the result is 0, the CRC check is passed. The candidate sequence is deleted and the check bit is used as the final decoding result.

[0097] When the instantaneous transmission capacity of the channel is lower than the transmission rate, or the signal-to-noise ratio is lower than the threshold When an interruption occurs, h is the channel state, and the interruption probability of the direct link is expressed as follows:

[0098]

[0099] like Figure 2 Figure 1 shows the relay laser communication coding principle of the FSO system of the present invention. At the source node, transmission diversity is achieved through collaboration, with different users sending different parts of the encoded codeword. This achieves diversity by sending different parts of the same information over two different channel links.

[0100] The purpose of channel coding is to improve the reliability of communication by increasing the redundancy of the information source. At the relay node, the first path is to multiply the sequence u1 by the generator matrix G1. The other path is to send m1 to the information selector, select the sequence m2 with a code length of k2, and then multiply the m2 sequence by the generator matrix G2 to obtain the sequence v1. The information sequence u1 obtained by the first path is added modulo 2 with the v1 obtained by the second path.

[0101] Finally, it is BPSK modulated and sent to the destination node. The signal-to-noise ratio γ from the source node to the destination node can be expressed as Where M is the optical degree of freedom, and the average signal-to-noise ratio of each channel is expressed as γ1=m1h SR ,γ2=m2h RD ,γ3=m3h SD , total transmission distance

[0102] The number of photons at the destination node is y D =h RD h SR y S G+h RD n R G+h RD n A +h SD n S +n D , where the gain G = m2 / (h SR m1+m R ), n A is the noise, m2 represents the average number of photons, h SR 、h RD 、h SD Indicates channel fading.

[0103] like Figure 3 Figure 2 shows a schematic diagram of relay laser communication coding in an FSO system. The subchannel for transmitting information bits is selected based on the Bhattacharyya parameter size and code rate. Channel coding is combined with cooperative diversity technology to transmit redundant information bits of the codeword to the destination node for verification, enabling the system to obtain more diversity gain and coding gain.

[0104] The process of combining and splitting the physical channel W is represented by the linear transformation matrix G N In the free-space laser relay communication system, which consists of a source node (SN), a destination node (DN), and a relay node (RN), the optical signal attenuation caused by atmospheric turbulence uses the Gamma-Gamma model, and the probability density function is defined as follows:

[0105]

[0106]

[0107] Where A represents the amplitude of the optical carrier, R represents the response coefficient, α represents the effective number of large areas during the scattering process, and β represents the effective number of small areas during the scattering process. is the Rytov variance.

[0108] ε means that the output of the event system is not equal to the original information sent by the source node, ε SR Indicates that the information decoded by the event relay node is not equal to the original information sent by the source node, ε RD represents the error transmission of the RD link, and the bit error rate of the coded cooperative communication is expressed as

[0109]

[0110] like Figure 4 Figure 2 shows an isosceles right triangle interleaving diagram. At the source node, the polar code output bits are fed into the interleaver in the order of the columns of the isosceles right triangle. This interleaving method can reduce storage space requirements.

[0111] use Indicates the jth output bit number of the interleaver, which is accumulated in columns. After the column is read, it goes to the next column, such as the output bit number case calculation; No. 1 No. 2 Calculate according to this principle.

[0112] like Figure 5 As shown in FIG, it is a schematic diagram of the interleaving module, wherein the cyclic permutation unit can realize cyclic permutation processing, the cyclic coefficient Q = 8, the bit permutation unit can optimize the receiving performance, and the codeword bits are mapped to the constellation bits, the number of constellation bits M = 5; then a block interleaver is used, and the interleaving matrix is ​​π = [π1,π2,…,π L ] T , change the information position to the maximum extent, resist long-film errors, yes A random permutation of , where π is the interleaving matrix.

[0113] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A relay laser communication coding method for FSO system, characterized in that the steps include: Step 1: Obtain the sequence to be encoded at the source node, and use K = k + m codewords as information bits for polar coding; The method for obtaining the sequence to be encoded and the information bits at the source node includes: S1, obtain the sequence to be encoded at the source node, determine the sub-channel for transmitting information bits according to the size of the Bhattacharyya parameter and the code rate, perform reliability sorting, and obtain the length Information sequence; S2, add on the information bit bit CRC check code, the obtained The bit codeword is polarization-encoded as the information bit of the polar code and sent to the encoder; Step 2: Use an interleaver to change the information position as much as possible to resist long-segment errors; Step 3: The codeword sequence at the source node is BPSK modulated and sent to the destination node and relay node; Step 4: The relay node performs SCL decoding on the received optical signal and extracts the information estimation sequence; Step 5: Use the Plotkin structure to construct the generator matrix of two codewords for polar coding; Step 6: The destination node combines the two signals with equal gain, deinterleaves them, and sends them to the decoder for error correction.

2. The relay laser communication coding method for FSO system according to claim 1, characterized in that: Methods for using an interleaver at the source node include: Step 1: After polarization encoding, the codeword sequence passes through a block interleaver to maximize the change of information position and resist long-segment errors. After BPSK modulation, the codeword sequence is sent to the channel. Step 2: Under the assumption of a turbulent channel, the error probability upper limit of the coding scheme is calculated, and the interleaving depth is adjusted based on the approximate error upper limit of the paired codewords.

3. The relay laser communication coding method for FSO system according to claim 1, characterized in that: The method of modulating a sequence at a source node and sending it to a destination node and a relay node includes: Step 1: Two different channels send different parts of the same information and extract the codewords through the information selector. , the codeword length is , after BPSK modulation, it is sent to the destination node; Step 2: The other side also modulates the codeword through BPSK and sends it to the relay node.

4. The relay laser communication coding method for FSO system according to claim 1, characterized in that: The polar coding method using the Plotkin structure to construct a generator matrix of two code words includes: Step 1: Construct the generator matrix of two code words for encoding. The first path is to generate the matrix Multiply to get the sequence , another path is to Send to the information selector, select the code length sequence ; Step 2: Sequences and Generator Matrices Multiply to get the sequence , the information sequence obtained by the first path The second path obtains Perform modulo-2 addition and finally perform BPSK modulation before sending to the destination node.

5. The relay laser communication coding method for FSO system according to claim 1, characterized in that: The destination node combines the two signals with equal gain, deinterleaves them, and sends them to the decoder for error correction. The methods include: Step 1: Receive the signal from the source node and the signal transmitted by the relay node , using equal gain merging; Step 2: Then send it to the deinterleaver to deinterleave the long continuous error bits, restore the disrupted interleaved sequence to the original sequence, and send the correct sequence after deinterleaving to the error correction code decoding module.

6. The relay laser communication coding method for FSO system according to claim 1, characterized in that: Methods for accomplishing error correction include: Step 1: Convert the long-slice errors caused by atmospheric turbulence into random errors, calculate the path metric value of each branch, select the largest path as the decoding result, and generate an estimated sequence of the information sequence; Step 2: Perform CRC check on the redundant information bits of the transmitted codeword at the same time, delete the check bits of the candidate sequence, and obtain the final decoding result.

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